General Instructions for NVSS Catalog Search

This WWW form allows the user to list selected portions of the NRAO/VLA Sky Survey (NVSS) radio source catalog. This survey was made with the National Radio Astronomy Observatory's Very Large Array telescope at a wavelength of 20 cm (1.4 GHz) and produced images of the sky north of declination -40 deg with a resolution of 45". The catalog contains sources with a peak brightness in excess of 2.5 mJy/beam. Both total intensity and linear polarization were imaged. The survey is now complete.

The source catalog consists of Gaussian model components fitted to the survey images and measures of the linearly polarized emission. If the source is significantly larger than the resolution of the survey (45") then it may be represented by several of these components. If requested (the default) the fitted parameters will be deconvolved from the instrumental resolution producing source sizes (or upper limits with 99% confidence) and the integrated flux density of the model. Error estimates for each of the source parameters are also given.

The source catalog contains a very large number of entries; in excess of 1,800,000. To limit the size of the output the region of the sky of interest should be limited. This form allows specifying the central Right Ascension and declination at one of several equinoxes and a circular ("Search radius"). All entries in this region meeting the other criteria are listed.

The details of the output listing depend on whether the raw, fitted, or deconvolved model parameters are requested. In the first two cases, the sizes are those in the image which include the instrumental response and the strength of the component is the peak surface brightness. In the case of deconvolved parameters, the size is that of, or an upper limit (99% confidence) to the source size and the strength is the integrated flux density of the component. Raw parameters are not corrected for known biases and no error estimates are given. The output listing gives the Right Ascension ("RA") and declination ("Dec") at the requested equinox. The distance from the specified central Right Ascension and declination ("Dist") is given in arcseconds. The position angle (from north through east) of the model component from the central position ("Ori") is given in degrees on the line under the separation. Next are the strength of the source ("Peak" or "Flux"), its major ("Major") and minor ("Minor") axis sizes and the orientation ("PA") of the major axis (in degrees from north through east) on the sky. Following this is a possible code ("Res", P*, R*, or S*) indicating structure more complex than can be fitted by the Gaussian model. If this field is nonblank then the model is inadequate to completely describe the source and the original image needs to be examined; in this case, the entry on the error line under the Res flag will give the offending value (P=peak, R=RMS, S=integrated) in 100s of microJy. The next entries in the output describe the linear polarization of the component. The first of these ("P_Flux") is the integrated linearly polarized flux density followed by the position angle of the "E" vectors ("P_ang) on the sky if the source was detected (1 sigma) in linear polarization. The final entries ("Field", "X_pix", "Y_pix") give the name of the original survey image field from which the component was derived and the "X" and "Y" (RA, dec.) pixel numbers of the center of the component. Below each parameter (except upper limits) is given the estimate of its standard deviation.

If only a single position is to be searched fill in the selection boxes in the sections marked "Single position to search". If searches on a number of positions is desired, then fill in the position list in the text box in the section labeled "List of positions to search".

Descriptions of individual fields on this form follow:


This is the equinox of the coordinates to be used for all input and output.


"Deconvolved" gives deconvolved component size, integrated flux density. "Fitted" gives fitted component size, peak flux density. The deconvolved parameters more accurately describe the actual source values. "Raw" is like "Fitted" except that no amplitude bias corrections are made and no error estimates are given.

Minimum peak flux density

Only sources with a peak brightness (mJy) higher than this value will be listed.

Minimum percentage polarization

Only sources with a percentage polarization higher than this value will be listed.


This string gives a label for this field in the output.

Central Right Ascension

The listing can be limited to a specified region in RA-Dec The center right Ascension at the specified equinox is given as hh mm ss.s. A search radius must also be specified.

Central declination

The listing can be limited to a specified region in RA-Dec The center declination is given as +/-dd mm ss.s. A search radius must also be specified.

Search radius

The radius of the specified circular region about the center Right Ascension and declination is given in arcseconds. Too large a value will increase the probability of a false association of a radio source and the target source; values larger than an arcminute (60") should be avoided if an accurate identification is desired. The default (blank or 0s) is 15".

List of positions

If many fields are to be searched, you may list their positions in a text box ("List of positions..."). This list contains one line per field, and each line has up to 5 logical columns containing, in order:
1. Field center right ascension as hh mm
2. Field center declination as +/-dd mm ss.s
3. (Optional) Search radius in arcsecond (defaults to 15")
4. (Optional) a zero - only needed if the field label is to be given 5. (Optional) Field label (defaults to blank)
An example of a position list entry is:
12 34 56.78 -00 12 34.5 15 0 My Star
This will search for NVSS sources within 15" of RA=12 34 56.78, Dec=-00 12 34.5.

Note: it may be most convienient to create this list in a text file and using the cut and paste facility of your system to enter the list into this box.